1. Field of the Invention
The present invention relates to a video signal processing circuit which performs analog-to-digital (AD) conversion of an analog composite video signal, such as a television broadcast signal, to obtain a digital composite video signal, and which performs video signal processing on the digital composite video signal.
2. Description of the Prior Art
Analog decoders using bipolar integrated circuits (BIP-IC) have been widely used as video decoders for generating an RGB signal from an analog composite video signal such as an analog television broadcast signal. With the advancement of digital technology in recent years, and with the widespread proliferation of digital displays such as liquid crystal displays (LCD) and plasma displays (PDP), there have been an increasing number of digital decoders (digital video decoders) using MOS-ICs.
Referring to FIG. 11, which is a block diagram showing the configuration of an analog video decoder 70, an externally-supplied analog composite video signal is input to the analog video decoder 70 and is supplied to an analog sync separation circuit 71 and to an analog video signal processing circuit 72. The analog sync separation circuit 71 separates a sync signal from the analog composite video signal and supplies the extracted sync signal to the analog video signal processing circuit 72. The analog video signal processing circuit 72 performs various video signal processes (including Y/C separation and color demodulation) on the analog composite video signal using the sync signal supplied from the analog sync separation circuit 71 to generate and output an analog RGB signal.
Referring to FIG. 12, which is a block diagram showing the configuration of a digital video decoder 80, an externally-supplied analog composite video signal is input to the digital video decoder 80, converted into a digital signal by an AD converter (ADC) 81, and thereafter supplied to a digital sync separation circuit 82 and to a digital video signal processing circuit 83. The digital sync separation circuit 82 separates a sync signal from the converted digital composite video signal and supplies the extracted sync signal to the digital video signal processing circuit 83. The digital video signal processing circuit 83 performs various types of video signal processing (including Y/C separation and color demodulation) on the digitized composite video signal using the sync signal supplied from the digital sync separation circuit 82 to generate and output a digital RGB signal.
Japanese Patent Laid-Open Publication No. 2003-153298 and Japanese Patent Laid-Open Publication No. Hei 10-254422 include description of analog composite video signals.
Accompanying digitization, a digital video decoder as described above has many advantages over an analog video decoder. A digital video decoder, however, is inferior to the analog video decoder in synchronization performance under a weak-electric-field condition. A difference in synchronization performance between the digital and analog video decoders will be described below.
FIG. 13A is a diagram showing the waveform of an analog composite video signal free from noise. The video signal shown in FIG. 13A is a black-and-white television signal composed of a luminance signal and a sync signal. A color television signal is formed by superimposing a color burst signal on a back porch and superimposing a carrier chrominance signal on the luminance signal of a black-and-white television signal.
Separation of the sync signal (sync separation) is performed by a comparator cutting off (slicing) the composite video signal at a slice level between a tip level of the sync signal (sync tip level) and a pedestal level, such as indicated by the dot-dash line in FIG. 13A. By this sync separation, the sync signal is separated, as shown in FIG. 13B. However, high-frequency noise exists on the composite video signal under a weak-electric-field condition, as shown in FIG. 13C. This noise affects sync separation.
FIG. 14 is a diagram showing sync separation procedures. An example of a procedure in the analog video decoder is shown at the left of FIG. 14, while an example of a procedure in the digital video decoder is shown at the right of FIG. 14. Comparison between the analog video decoder and the digital video decoder will be made with reference to FIG. 14 with respect to sync separation under a weak-electric-field condition.
The analog video decoder removes high-frequency components from the analog composite video signal by using an analog low-pass filter and cuts off the obtained signal at a slice level by means of a comparator to separate the sync signal, as shown in the left section of FIG. 14. Thus, the analog video decoder is capable of suitably separating the sync signal even under weak-electric-field conditions.
On the other hand, the digital video decoder converts the analog composite video signal into a digital composite video signal by sampling the analog composite video signal based on a sampling clock, as shown at the right of FIG. 14. The composite video signal after this analog-to-digital conversion contains a digitized noise component due to noise superimposed on the weak-electric-field signal. This noise component can be reduced to a certain level by a digital low-pass filter but finally remains in the signal after sync separation, i.e., the reproduced sync signal.
As described above, the digital video decoder is inferior to the analog video decoder in terms of sync separation under a weak-electric-field condition at the present time.
The sync signal is information on timing of the composite video signal. Therefore, a problem that the video frame shakes and a problem that the luminance signal or color signal is not correctly reproduced arise if the sync signal is not accurately reproduced.